Recently, there has been a growing interest in the development of thin film piezo-electric materials for use in all fibre acousto-optic modulators. In FIG. 1 there is illustrated a schematic representation of the typical modulator device structure 10 where a film of active material 11 (typical thickness from 5 to several 10's of μm) is sandwiched between two electrical contact layers 12, 13 (thicknesses up to 1 μm) covering a full 360° surface of a fibre. The cylindrical geometry of such devices 10 results in highly efficient polarisation independent modulators as the acoustic waves are focussed at the fibre core 15.
The active material 11 can be Zinc Oxide (ZnO) which is a II-VI semiconductor with strong piezo-electric and electro-optic properties ideal for use in compact thin film fibre modulators with frequency responses up to 1 GHz. In crystalline zinc oxide, the c-axis is a polar axis due to effective ionic charges between the alternating Zn and O layers. It is therefore important that the ZnO thin film structure is one in which the crystallites are oriented with their c-axis parallel to the applied electric field.
To date ZnO films used in the fabrication of all-fibre modulators have been deposited using various forms of sputtering from a ZnO target. The devices previously constructed have had limited maximum attainable efficiency and phase modulation. Further, with utilising sputtering, rotation of the optical fibre was required due to the directional nature of the high energy deposition process. This was found to have an undesirable affect on device performance. Further, the most important limiting factor in these devices appears to be the non-negligible conductivity of the deposited films.